Method of combustor cycle airflow adjustment

ABSTRACT

A method of combustor cycle air flow adjustment for a gas turbine engine according to the present invention solves the problem of a higher flame temperature in the combustor, thereby affecting the emission levels when a heat-recuperated air flow cycle is used to increase the compressed air temperature. In low emission combustors this impact is severe because emission levels are significantly dependent on the primary combustion zone flame temperature. The method of the present invention includes a step of changing a geometry of an air flow passage and thereby changing distribution of a total air mass flow between an air mass flow for combustion and an air mass flow for cooling in order to ensure that flame temperature in a primary combustion zone of a combustor are maintained substantially the same whether the gas turbine engine is manufactured to operate as a simple air flow cycle engine or as a heat-recuperated air flow cycle engine. In an embodiment of the present invention, the changing of the geometry of the air flow passage by changing the number and size of perforations in an impingement cooling skin so that with minimal changes the impingement cooling skin serves duel purposes both as a cooling device for cooling the combustor wall and as a valve means for combustor cycle air flow adjustment, which makes the method simple and economical.

FIELD OF THE INVENTION

[0001] The present invention relates to gas turbine engines,particularly to air flow distribution adjustment of gas turbine enginesto ensure that low emissions not be affected by air flow cycle patternsof the engines whether the engine is designed as a simple air flow cycleor a heat-recuperated air flow cycle.

BACKGROUND OF THE INVENTION

[0002] Industrial gas turbine engines are subject to increasinglystringent emission requirements. In order to provide a marketable powergeneration product, an engine producing the lowest possible emissions iscrucial. Emissions of nitrogen oxides (NO_(x)) and carbon monoxide (CO)must be minimized over specified engine operating ranges. To achievethis low level of emissions the combustion system requires the completeburning of fuel and air at low temperatures.

[0003] Combustors that achieve low NO_(x) emissions without waterinjection are known as dry-low emissions (DLE) and offer the prospect ofclean emissions combined with high engine efficiency. This technologyrelies on a high air content in the fuel/air mixture. While lowemissions are critical, it is also important to minimize the cost ofmanufacturing and maintaining the combustion system, if a viable productis to be realized.

[0004] With regard to another aspect of the gas turbine engine, engineefficiency is always of critical concern. It is known that gas turbineefficiency can be substantially increased by recuperating heat from theengine exhaust. In a gas turbine engine using a heat-recuperating airflow cycle, compressor air is passed through a recuperator or heatexchanger attached to the engine exhaust end before entering thecombustor. The resulting higher combustor inlet air temperature thusrequires less fuel burn to achieve the same exit gas temperaturecompared with a gas turbine engine using a simple air flow cycle inwhich the compressor air enters the combustor directly and unaltered.The result of the heat-recuperated air flow cycle is significantlyimproved gas turbine cycle efficiency. The change in combustor inlet airtemperature and result in a combustor fuel/air ratio shifting to achievethe same exhaust gas temperature. In low emission combustors the impactof fuel/air ratios shifting is the most severe because emission levelsare intensely dependent on primary combustion zone fuel/air ratios.Adjusting the combustor geometry will compensate for the effect offuel/air ratio change between combustors of gas turbine engines usingsimple and heat-recuperated air flow cycles. However, it can be costprohibitive to adapt a gas turbine engine model which operates as asimple air flow cycle design, to be operable as a heat-recuperated airflow cycle design, or vice versa.

[0005] Therefore, there is a need to develop a method of engine designto cost effectively overcome the effect of the combustor fuel/air ratiochanging between simple and recuperated air flow cycles when adapting agas turbine engine model to be operable as either a simple or aheat-recuperated air flow cycle design while achieving low emissionlevels.

SUMMARY OF THE INVENTION

[0006] One object of the present invention is to provide a method of lowemission engine design for cost effectively overcoming the effect of acombustor fuel/air ratio change between simple and recuperated air flowcycles of the combustor engine.

[0007] Another object of the present invention is to provide a method ofcombustor cycle air flow adjustment for a gas turbine engine to ensurethat the flame temperature is maintained substantially the same whetherthe gas turbine engine utilizes a simple air flow cycle or aheat-recuperated air flow cycle.

[0008] A further object of the present invention is to provide animpingement cooling skin for a gas turbine engine combustor which servesdual purposes as a cooling device to cool the combustor wall and as avalve to adjust distribution of a total air mass flow between an airmass flow for combustion and an air mass flow for cooling, therebychanging the fuel/air ratio to ensure that the combustor flametemperature is maintained substantially the same whether the gas turbineengine operates as a simple air flow cycle engine or a heat-recuperatedair flow cycle engine.

[0009] In accordance with one aspect of the present invention, a methodof combustor cycle air flow adjustment for a gas turbine engine designis provided. The method comprises a step of changing a geometry of anair flow passage and thereby changing distribution of a total air massflow between an air mass flow for combustion and an air mass flow forcooling, in order to ensure that a flame temperature in a primarycombustion zone of a combustor is maintained substantially the samewhether the gas turbine engine operates as a simple air flow cycleengine or a heat-recuperated air flow cycle engine.

[0010] The geometry of the air flow passage is preferably changed todecrease the air mass flow for combustion when the gas turbine engineuses a heat-recuperated air flow cycle, compared with the air mass flowfor combustion when the gas turbine engine uses a simple air flow cycle.It is preferable that the changing of the air flow passage geometry isachieved by changing a geometry of an impingement skin of the combustor.

[0011] In accordance with another aspect of the present invention, animpingement cooling skin in combination with a gas turbine enginecombustor is provided. The impingement cooling skin is attached to thecombustor and comprises a first group of holes therein adjacent to acombustor wall section defining a primary combustion zone, the numberand size of the holes of the first group being predetermined tosubstantially meet a cooling requirement for the combustor. Theimpingement cooling skin further includes a second group of holestherein adjacent to a combustor wall section defining a secondarycombustion zone, the number and size of the holes of the secondary groupbeing adjusted to substantially meet a flow distribution requirementwhether the gas turbine engine operates as a simple air flow cycleengine or a heat-recuperated air flow cycle engine.

[0012] The present invention advantageously provides a cost effectivesolution to overcome the effect of combustor fuel/air ratio changebetween a simple air flow cycle and a heat-recuperated air flow cycle ofthe gas turbine engine operation. Without any added parts, theimpingement cooling skin of the combustor, according to the presentinvention, serves dual purposes as a cooling device to cool thecombustor wall and as a valve to adjust the distribution of the totalair mass flow between the air mass flow for combustion and the air massflow for cooling such that the required air mass flow for maintainingthe combustor flame temperature substantially the same, is achievedregardless of the engine air flow cycle pattern. Thus, low emissions ofthe gas turbine engine are ensured.

[0013] Other advantages and features of the present invention will bebetter understood with reference to a preferred embodiment describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Having thus generally described the nature of the presentinvention, reference will now be made to the accompanying drawings, byway of example showing a preferred embodiment, in which:

[0015]FIG. 1 is a schematical illustration of a gas turbine engineoperating in a simple air flow cycle;

[0016]FIG. 2 is a schematical illustration of a gas turbine engineoperating in a heat-recuperated air flow cycle;

[0017]FIG. 3 is a graphical representation of the relationship betweenNO_(x) emission levels and the flame temperature;

[0018]FIG. 4 is a diagram of a air flow distribution of a gas turbineengine incorporating the present invention; and

[0019]FIG. 5 is an elevational side view of a gas turbine combustor withan impingement cooling skin attached thereto according to one embodimentof the present invention, a section of the combustor being cut away toshow the cross-section of the combustor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] A simple air flow cycle of a gas turbine engine operation isschematically illustrated in FIG. 1, in which the gas turbine engine,generally indicated at numeral 10, includes a housing 12 having an airinlet 14 and a gas exhaust outlet 16. Within the housing 12 a compressor18 is driven by a turbine 20 through a shaft 22. A combustor 24 isprovided to generate combustion gases in order to rotate the turbine 20and further rotate a power turbine 25 which in turn drives a generator26 through a shaft 28. The air flows in the simple air flow cycle of thegas turbine engine operation are indicated by arrows 30. Air enteringthe inlet 14 is compressed by the compressor 18 and the compressed airthen directly enters the combustor 24 for combustion. The combustiongases discharged from the combustor 24 are directed through the turbine20 and the power turbine 25, and then the exhausted gases are dischargedfrom the gas exhaust outlet 16.

[0021] A heat-recuperated air flow cycle of a gas turbine engineoperation is schematically illustrated in FIG. 2, in which the engine,generally indicated at numeral 11 is similar to the gas turbine engine10 of FIG. 1. The gas turbine engine 11 includes parts similar to thoseof gas turbine engine 10, which are indicated by similar numerals andwill therefore, not be redundantly described herein. Additionally, gasturbine engine 11 includes a recuperator 32 which is a heat exchangerattached to the gas exhaust outlet 16 to recover heat from the exhaustedgases. Air flows in the heat-recuperated air flow cycle of the gasturbine engine operation are indicated by arrows 34. The air enteringthe air inlet 14 is compressed by the compressor 18. However, instead ofdirectly flowing into the combustor 24, the compressed air is directedto the recuperator 32 for heat exchange and then enters the combustor24. Similar to the simple air flow cycle shown in FIG. 1, the combustiongases are discharged from the combustor 24 to rotate the turbine 20 andthe power turbine 25 and then the exhausted gases are discharged fromthe gas exhaust outlet 16. The recuperator 32 transfers heat from theexhausted gases in the gas exhaust outlet 16 to the compressed air andtherefore, the temperature of the compressed air entering the combustor24 is much higher than the temperature of the compressed air whichenters the combustor 24 in the simple air flow cycle as schematicallyillustrated in FIG. 1. Higher compressed air temperatures make thecombustion operation more efficient, since less fuel is burned toachieve the same exhaust gas temperature.

[0022] Nevertheless, lower compressed air temperatures increase theamount of fuel to be burned and the fuel/air ratio in the combustor 24becomes richer. It is already known that NO_(x) emissions depend onflame temperatures as illustrated in FIG. 3, and it is desirable tomaintain the flame temperature at a low point to meet the low NO_(x)emission requirements. In a DLE system, the low flame temperature pointis achieved by maintaining a very lean fuel/air ratio in the combustor,particularly in the primary combustion zone. Generally the lean fuel/airratio in the combustor cannot be maintained when a gas turbine enginedesign is altered from that of a simple air flow cycle operation to thatof a heat-recuperated air flow cycle operation, because more fuel mustbe burned to achieve the same exhaust gas temperature. Adjusting thecombustor geometry will compensate for the effects of the switch betweensimple and heat-recuperated air flow cycles of a gas turbine engineoperation. However, changing the combustor geometry is cost prohibitiveon engine designs when it is desired to adapt a gas turbine engine modelwhich operates as a simple air flow cycle design, to be operable as aheat-recuperated air flow cycle design, or vice versa.

[0023] A method of combustor cycle air flow adjustment for a gas turbineengine is therefore developed according to the present invention, and isgenerally illustrated in FIG. 4 in which a total air mass flow 36 of agas turbine engine is distributed between an air mass flow 38 for acombustion system 42 of the engine and an air mass flow 40 for a coolingsystem 44 of the same engine. The exhausted combustion gases withexhausted cooling air are discharged from the engine as indicated byarrow 46. The temperature of the exhausted combustion gases must remainconstant, independent of cycle. If resistance to either air mass flow 38or 40 is changed, the distribution of the total air mass flow 36 betweenthe air mass flow 38 for the combustion system 42 and the air mass flow40 for the cooling system 44 will be changed accordingly. The change ofair mass flow resistance can be achieved with a valve means 48 which maybe connected to either air flow branch, connected to the air flow branchfor the air mass flow 40.

[0024] According to the present invention there is no need for the valvemeans 48 physically presented herein, to adjust the air flow resistancein either air flow branch. Changing the geometry of an air passage foreither air mass flow 38 or 40 will perform a virtual valve function,regulating the air flow resistance of the air passage for the air massflow 38 or 40, thereby resulting in adjustment of the air flowdistribution. When the engine is to be designed for a heat-recuperatedair flow cycle operation and that design is adapted from an enginedesign for a simple air flow cycle operation, changing the geometry ofthe air passage either for the air mass flow 38 or the air mass flow 40in order to decrease the air mass flow 38, will compensate for thedecreased fuel burn, thereby maintaining the flame temperature in theprimary combustion zone of the combustor and exhaust gas temperature,substantially the same whichever original engine design is used.

[0025] In accordance with one embodiment of the present invention, it isconvenient and cost effective to change the geometry of an impingementcooling skin to affect the air mass flow 38 for the combustion system 42which is illustrated in FIG. 5. A cyclone combustor 110 includes acylindrical combustor can 112 having a central axis 114, an upstream end116 and a downstream end 118 defined by an annular side wall 120. Theupstream end 116 is closed by an upstream end wall 122 and thedownstream end 118 is in fluid communication with a turbine section ofthe engine (not shown). Three entry openings 124 (only one is shown) areprovided in the annular side wall 120 adjacent to the upstream end wall122 for receiving premixed fuel/air mixture into the combustor can 112.The combustion processing of the premixed fuel/air mixture takes placegenerally in a primary combustion zone 126 which is defined within anupstream section of the combustor can 112. The combustion productsgenerated within the primary combustion zone 126, as well as theun-reacted fuel and air will complete the combustion process in asecondary combustion zone 128 which is a section of the combustor can112 downstream of the primary combustion zone 126. The final combustionproducts are then discharged from the downstream end 118 into theturbine.

[0026] Three fuel and air premixing tubes 130, such as venturi premixingtubes,(only two are shown) are attached to the side wall 120 of thecombustor can 112 and are positioned adjacent to the upstream end wall122. The premixing tubes 130 are circumferentially, equally spaced apartfrom one another and are in fluid communication with the combustor can112 through the respective entry openings 124 in the side wall 120.

[0027] Each premixing tube 130 includes a major tube section 132 forproducing the fuel/air mixture therein and an outlet section 134 forinjecting the fuel/air mixture into the combustor can 112 forcombustion. The major tube section 132 has a central axis 136 thereofextending substantially parallel to the central axis 114 of thecombustor can 112. The outlet section 134 has a central axis 138 thereofextending substantially perpendicular to the central axis 136 of themajor tube section 132 and is oriented toward the combustor can 112radially with a tangential offset. Thus, lean fuel/air mixture flowsinjected from the respective entry openings 124 in the side wall 120create a swirling helical pattern within the primary combustion zone 126of the combustor can 112 as a result of the tangential offset of thelean fuel/air mixture flows exiting from the outlet sections 134 of thepremixing tubes 130, respectively. The swirling helical pattern of theburning lean fuel/air mixture in the primary combustion zone 126provides optimum circulation of low temperature flames in the combustorcan 112 which improves the liner life span of the combustor can 112,flame stability in the combustion process and engine turn-down, as wellas the reduction of combustion noise and emission levels.

[0028] The size of the tube advantageously inhibits flashback from theprimary combustion zone 126 into the major tube section 132 of thepremixing tube 130. Pilot fuel lines 142 and ignitors 146 are placed totake advantage of the locations of the entry openings 124 and thetangential direction of the lean fuel/air mixture flow momentumgenerated from the tangential offset of the premixing tubes 130.

[0029] An impingement cooling skin 148 is provided. The impingementcooling skin 148 is made of a wrap-around sheet metal with perforationstherein and is positioned around the annular side wall 120 of thecombustor can 112 in a radially spaced apart relationship. It isoptional that the impingement cooling skin 148 includes a perforated endskin 149 positioned axially spaced apart from the upstream end wall 122of the combustor can 112.

[0030] The perforations in the impingement cooling skin 148 as well asin the end skin 149 are formed in two groups. The first group ofperforations includes holes 150 in the end skin 149 and in a section ofthe impingement cooling skin 148 adjacent to a combustor wall sectiondefining the primary combustion zone 126. A second group of perforationsincludes holes 152 in a section of the impingement cooling skin 148adjacent to a combustor wall section defining the secondary combustionzone 128. The first group of holes 150 are distributed in apredetermined pattern with a predetermined number and a predeterminedsize thereof to substantially meet the cooling requirements of thecombustor wall section defining the primary combustion zone 126. Thesize and number, as well as the distribution pattern of the second groupof holes 152 are subject to change when the cyclone combustor 110 isdesigned for different air flow cycle operations, in order tosubstantially meet the flow distribution requirements relating to theparticular air flow cycle of a gas turbine engine operation, as well asto provide secondary zone cooling.

[0031] The impingement cooling skin 148 further includes a sealing ring154 which is sealingly connected to a housing 156, only a section ofwhich is shown in FIG. 5. The housing 156 contains the combustor can112, and an annulus 158 is thereby formed therebetween as a section ofthe air passage for cooling air. When compressed air approaches thecyclone combustor 110 from above, the total air mass flow is distributedbetween the air mass flow 38 which enters the premixing tubes 130 to mixwith fuel to produce the fuel/air mixture for combustion in thecombustor can 112, and the air mass flow 40 which enters holes 150 inthe end skin 149 and holes 150 and 152 in the impingement cooling skin148 by way of the annulus 158. The air entering the holes 150 in the endskin 149 and holes 150 and 152 in the impingement cooling skin 148impinges upon the upstream end wall 122 and side wall 120 of thecombustor can 112 to remove the heat therefrom. The combustor wallsection defining the primary combustion zone 126 is generally exposed tohigher temperatures than the combustor wall section defining thesecondary combustion zone 128, therefore the combustor wall sectiondefining the primary combustion zone 126 is cooled by the air from theholes 150 which are specifically designed to substantially meet thecooling requirements regardless of the air flow cycles of the gasturbine engine operation.

[0032] In contrast to the predetermined pattern of the holes 150, theholes 152 are designed substantially in accordance with the air flowdistribution requirements for the particular air flow cycles of gasturbine engine operation. When the combustor 110 is designed for asimple air flow cycle operation, the number and size of the holes 152are decreased to increase the air resistance of the cooling air passagerelative to the air mass flow distribution for a heat-recuperated airflow cycle operation, such that less air mass flow 40 is directed forcooling while relatively more air mass flow 38 is directed to thepremixing tubes 130 for combustion.

[0033] On the other hand, when the combustor 110 is designed for aheat-recuperated air flow cycle the number and size of the air holes 152are increased to reduce the air resistance in the cooling air passagesuch that relative to the air mass distribution for the simple air flowcycle operation, more air mass flow 40 is directed for cooling andrelatively less air mass flow 38 is directed into the premixing tubes130 for combustion.

[0034] In the embodiment of the present invention, by using such amethod, the impingement cooling skin 148, 149 serves duel purposes bothas a cooling device to cool the combustor wall 120, 122 and as a valvemeans to adjust air distribution flow between air mass flows 38, 40 forcombustion. Thus, it is convenient and cost effective to just modify theimpingement cooling skin 148, 149 design without need for othercomplicated changes of the combustor in order to provide a gas turbineengine design for either simple or heat-recuperated air flow cycleoperation while maintaining emissions at a same low level. Changing thenumber and size of holes 152 of the impingement cooling skin 148 willalso affect cooling results. However, the holes 152 are designated forair impingement upon the combustor wall section defining the secondarycombustion zone 128 which is exposed to lower temperatures and thereforethe negative effect of cooling is reduced. The cooling aspect isoptimized for the more severe conditions related to heat-recuperatedcycles.

[0035] Optionally, a gas turbine engine combustor may be manufacturedwith a removable cover sheet or ring to selectively cover the holes 152of the impingement cooling skin 148, according to a further embodimentof the present invention. Thus, the gas turbine engine is adapted tooperate alternatively as a simple or heat-recuperated air flow cycleengine while maintaining low emission levels.

[0036] The method of combustor cycle air flow adjustment for a gasturbine engine according to the present invention is applicable tovarious combustor systems, and the cyclone combustor described above isan example only, illustrating a particular embodiment of the presentinvention. This invention is also applicable to swirled diffusion dumpcombustion designs. Modifications and improvements to theabove-described embodiment of the present invention may become apparentto those skilled in the art. The foregoing description is intended to beexemplary rather than limiting. The scope of the invention is thereforeintended to be limited solely by the scope of the appended claims.

I/We claim:
 1. A method of combustor cycle airflow adjustment for a gasturbine engine design comprising a step of changing a geometry of an airflow passage and thereby changing distribution of a total air mass flowbetween an air mass flow for combustion and an air mass flow for coolingto ensure that a flame temperature in a primary combustion zone of acombustor is maintained substantially the same whether the gas turbineengine operates as a simple air flow cycle engine or as aheat-recuperated air flow cycle engine.
 2. The method as claimed inclaim 1 wherein the geometry of the air passage is changed to decreasethe air mass flow for combustion when the gas engine is used in aheat-recuperated air flow cycle, with respect to the air mass flow forcombustion when the engine is used as a simple air flow cycle engine. 3.The method as claimed in claim 2 wherein the changing of the air flowpassage geometry is achieved by changing a geometry of a cooling airpassage.
 4. The method as claimed in claim 2 wherein the changing of theair flow passage geometry is achieved by changing a geometry of animpingement skin of the combustor.
 5. The method as claimed in claim 2wherein the changing of the air flow geometry is achieved by changingthe number and size of a plurality of small holes in an impingement skinof the combustor such that the impingement skin performs as both acooling device and a valve for air mass flow distribution adjustmentbetween the simple flow cycle and the heat-recuperated flow cycle. 6.The method as claimed in claim 5 wherein the impingement skin comprisesa first-group of holes therein adjacent to a combustor wall sectiondefining the primary combustion zone, the number and size of the holesof the first group being predetermined to substantially meet a coolingrequirement for the combustor; and a second group of holes thereinadjacent to the a combustor wall section defining a secondary combustionzone, the number and size of the holes of the second group beingadjusted to substantially meet a flow distribution requirement when theair flow cycle of the gas turbine engine is changed between the simpleflow cycle and the heat-recuperated flow cycle.
 7. An impingementcooling skin in combination with a gas turbine engine combustor to whichthe impingement cooling skin is attached, the impingement skincomprising a first group of holes therein adjacent to a combustor wallsection defining a primary combustion zone, the number and size of theholes of the first group being predetermined to substantially meet acooling requirement for the combustor; and a second group of holestherein adjacent to a combustor wall section defining a secondarycombustion zone, the number and size of the holes of the second groupbeing adjusted to substantially meet a flow distribution requirementwhen an air flow cycle of the gas turbine engine is changed between asimple air flow cycle and a heat-recuperated air flow cycle.